Question: Which of these numbers is composite? ${2,\ 17,\ 19,\ 81,\ 83}$
Explanation: A composite number is a number that has more than two factors (including 1 and itself). 2, 17, 19, and 83 each have only two factors. The factors of 81 are 1, 3, 9, 27, and 81. Thus, 81 is the composite number.